The present invention relates to a method for selectively etching an intended part of an object being processed with a fluid, which is kept in liquid phase by heating and pressuring the fluid, and also relates to an etching apparatus.
A process of etching a silicon nitride film using hot phosphoric acid, for example, is one of known methods for selectively etching just one of multiple layers stacked over a substrate (W. van Gelder and V. E. Hauser, J. Electrochem. Soc., 144, 869 (1967)). This etching method utilizes hot phosphoric acid because the acid etches a silicon nitride film at a rate much higher than a silicon dioxide film. Therefore, according to this etching method, only the silicon nitride film can be selectively etched away with the silicon dioxide film left. In this case, a selectivity obtained by the etching process using hot phosphoric acid is as high as 30 to 50, whereas a selectivity obtained by a dry etching process is only about 15. Accordingly, this type of etching process has been used widely in the fabrication process of semiconductor devices.
However, if this known selective etching process using hot phosphoric acid, for instance, is performed, the following problems arise.
Firstly, hot phosphoric acid results in a low etch rate. If a silicon nitride film is etched using hot phosphoric acid heated up to about 150xc2x0 C., for example, an etch rate of just about 4 nm/min is attainable. In this case, a silicon dioxide film can be etched with the hot phosphoric acid at a rate of about 0.13 nm/min and the etch selectivity of silicon nitride to silicon dioxide is about 31. That is to say, the etch selectivity is high to a certain extent but the etch rate is low. To the contrary, if the etching process is performed using hot phosphoric acid heated up to about 180xc2x0 C., the etch rate of the silicon nitride film can be increased up to about 10 nm/min but the etch selectivity decreases to about 10. That is to say, it is hard to attain both a high etch rate and a high etch selectivity.
Secondly, if an acid such as phosphoric acid is disposed of as it is, the environment might be polluted in various ways. For example, disposal of phosphoric acid causes eutrophication of rivers or seawater. Therefore, phosphoric acid should be treated before it is disposed of, but a complete treatment rises the treatment cost. It is also known that if a silicon dioxide film is selectively etched using hydrofluoric acid, for example, a serious problem might arise. For example, disposal of the hydrofluoric acid withers plants. Generally speaking, it is natural that substances such as acids and alkalis, exhibiting some etch capability, might damage the tissues of human bodies, animals, plants and so forth.
Further, when a dry etching process is performed, the resultant etch selectivity is not higher than 15 and the resultant etch rate is only about 10 nm/min. Also, in this dry etching process, even a semiconductor substrate, which is located under an object being processed and should not be etched, is damaged unintentionally due to exposure to plasma ions, thereby possibly causing defects such as dislocations.
Water, carbonic acid gas and so on are fluids exhibiting very low etch capability in a normal state but does not affect the environment so much. However, we found that even a fluid like this shows unique etch capability in a special state if the fluid is heated and pressured under some specific conditions. It is therefore an object of the present invention to provide etching method and apparatus for selectively and efficiently removing a target layer from an object being processed by utilizing this phenomenon.
An inventive etching method is a method for etching a target layer of an object being processed. The method includes the steps of: heating and pressuring a fluid under such conditions that the fluid is a liquid; and exposing the target layer to the fluid and allowing a substance in the fluid to react with the target layer, thereby etching away the target layer.
According to this method, only a target layer of an object being processed can be selectively etched away with a certain fluid at a high rate by heating and pressuring the fluid, even though the fluid does not exhibit that high etch rate at a normal temperature and at a normal pressure. For example, a fluid such as water or carbon dioxide hardly affects the environment at a normal temperature and at a normal pressure, and the treatment cost of the fluid used is very low. The etch capability of a fluid under some specific conditions is greatly different from that of the fluid in a normal state. Accordingly, by utilizing this phenomenon, the etching process can be performed efficiently as a whole.
When the etching process is performed using a water as the fluid, a target layer may be exposed to the water that is a roughly subcritical liquid.
Particularly where the target layer is a silicon nitride film or a crystalline silicon layer, the water is preferably heated to a temperature between 150xc2x0 C. and 315xc2x0 C. and preferably pressured to a vapor pressure or higher. The water is more preferably heated to a temperature between 150xc2x0 C. and 250xc2x0 C.
Also, where the target layer is a silicon nitride film or a crystalline silicon layer, at least a silicon dioxide film may be used as an etch stopper. In that case, a selective etching process can be performed because a water as a subcritical liquid can etch the silicon nitride film at a higher rate than the silicon dioxide film.
An inventive apparatus etches a target layer of an object being processed using a fluid. The apparatus includes: a vessel in which the object is loaded and subjected to an etching process; a fluid supplier for supplying the fluid into the vessel; means for separating the fluid, drained from the vessel, from a substance resulting from the etching process; a reservoir for storing the fluid that has been separated by the separating means; and means for circulating the fluid from the reservoir back to the fluid supplier.
This apparatus can recycle a fluid once used after the composition and purity of the fluid have been adjusted appropriately for an etching process. As a result, the cost and time for the etching process can be further reduced.
Where the fluid is a water, the separating means may vaporize the water to separate the water from the substance resulting from the etching process.
Also, the separating means may condense the substance resulting from the etching process to separate the substance from the fluid.